As hard disc recording and reproduction systems for digital data processing have evolved, there have been continuing increases in track density and longitudinal recording density, such that data recording capacities have increased by orders of magnitude. A fundamental factor in achieving these results has been the development of transducers which are supported by air bearings at very small flying heights (1 microinch or less) above the surface of the disc. The aerodynamics of the pad facing the disc, and a sensitive and precise gimbal support arm, facilitate noncontact operation with these minute gaps which in turn provides extremely efficient coupling between the transducer and the active surface (whether magnetic or magneto-optical) of the disc.
As advances have been made in these respects, corollary advances have also been made in disc manufacture, and in manufacturing processes and test procedures, to enable the disc surfaces to be essentially planar, to a high degree of precision. The discs are mass produced, enabling the virtually universal adoption of hard disc files for data processors in small and large capacity systems to become feasible because of the very low cost and very high performance levels which have been reached. The discs may be single sided or double sided, as they are burnished and finished to a given smoothness. With submicroinch flight heights, however, burnishing alone is not satisfactory, because very minor irregularities, typically called asperities, still can exist. These must either be eliminated before the disc can be installed, or the disc must be rejected for use.
Automated test beds have been devised for use in a final honing procedure for these high capacity hard discs. These test beds include "glide head" mechanisms, each glide head having a sensitive force sensor so that, with the glide head flying above the disc surface at a given height (at the order of a microinch or less) asperities can be detected. The disc is rotated at angular velocities typical for normal operations, giving surface rates of 400 ips to 600 ips, depending upon radial position. The glide head is scanned across the active recording surface of the disc, with the sensor generating a signal excursion whenever an asperity is encountered. Depending on the amplitude and duration of the signal excursion, these asperities can be categorized (as for example "hard" hits or "soft" hits) and the instrumentation system can identify the radius for future processing. At this point, a honing head, supported by a gimbal arm to be radially movable, is also scanned across the disc, specifically to those radial positions at which asperities had been detected. The honing head typically has small projections from a flat surface, and flat contact areas on the projections that are separated by grooves, such that the edges of the projections engage and hone the asperities.
Such honing head designs, however, have disadvantages that become most apparent when attempting to provide a surface for recording/reproduction head operation at submicroinch levels. Highly polished flat surfaces in contact induce forces of molecular attraction between them, and thus introduce stiction effects which can render operation non-uniform. In addition, with a honing head of this design the top increment of an asperity may be separated from its base, but is not necessarily removed from the disc itself, thus representing an object that can possibly interfere with signal transduction.
After honing head operation, the disc is again tested by the glide head and instrumentation system, to verify that asperities beyond the chosen threshold have been eliminated. Thereafter, the disc can be approved for use in a production unit. These processes are carried out in clean room conditions and the discs are confined within closed environments with air properly filtered to remove all but very minute particle sizes which can be tolerated.